The present invention relates to processes for producing hafnium tetra-tertiary-butoxide Hf(OC4H9)4, tetrakis(acetylacetonato)hafnium Hf(C5H7O2)4, tetrakis(1-methoxy-2-methyl-2-propanolato)hafnium Hf(C5H11O2)4, hafnium tetra-tertiary-amyloxide Hf(OC5H11)4, tetrakis(3-methyl-3-pentoxy)hafnium Hf(OC6H13)4, and tetrakis(hexafluoroacetylacetonato)hafnium Hf(C5F6HO2)4, which are regarded as promising hafnium film-forming materials of hafnium-series insulating films (e.g., HfO2 and HfSiON), which attract much attention as the next generation high dielectric constant gate insulating films in semiconductor production.
Hitherto, SiO2 has been used for a long time for gate insulating films in semiconductor production. This is because it was possible to respond to the trend toward finer devices along with higher integration of semiconductors by making SiO2 films thinner. In recent years, however, the trend toward finer devices has grown further in order to achieve higher functionality and higher integration of LSI. As a result, physical limit is getting closer in making SiO2 films thinner, and it is now difficult to respond to the trend toward further finer devices. Thus, hafnium-series insulating films attract much attention as gate insulating films that are substitutes for SiO2 films. Hafnium-series insulating films have dielectric constants several times higher than that of SiO2, and it is possible for that to increase the physical film thickness. Therefore, hafnium-series materials are those capable of responding to the trend toward finer devices.
To form such hafnium-series insulating films, it is possible to cite physical vapor deposition (PVD) and chemical vapor deposition (CVD). In general, it is difficult in PVD to form a uniform film on an uneven substrate and to control the film composition. In CVD, however, it is possible to form a uniform film on substrate irrespective of whether the substrate has unevenness or not, and it is superior in controlling the film composition. In forming gate insulating films, it may be necessary to form a uniform film on an uneven portion, although it may depend on the process of the gate stack production. Furthermore, it is important to control the film composition, since the film composition affects electric characteristics of semiconductor. Therefore, it is a current mainstream to use CVD for forming gate insulating films.
It is necessary to provide a hafnium film-forming raw material that has a high vapor pressure in order to form a hafnium-series insulating film by CVD. As a hafnium complex that is a hafnium film-forming raw material that has a high vapor pressure, it is necessary to have a bulky substituent in order to prevent the bonding intermolecular interaction (e.g., crosslinking coordinate bond) and to have a small molecular weight. As hafnium film-forming materials that have such necessities and attract attention in recent years, there are hafnium complexes such as hafnium tetra-tertiary-butoxide Hf(OC4H9)4, tetrakis(acetylacetonato)hafnium Hf(C5H7O2)4, tetrakis(1-methoxy-2-methyl-2-propanolato)hafnium Hf(C5H11O2)4, hafnium tetra-tertiary-amyloxide Hf(OC5H11)4, tetrakis(3-methyl-3-pentoxy)hafnium Hf(OC6H13)4, and tetrakis(hexafluoroacetylacetonato)hafnium Hf(C5F6HO2)4. These six hafnium complexes respectively have relatively high vapor pressures of 90° C./6.5 Torr, 82° C./0.001 Torr, 135° C./7.6 Torr, 125° C./3 Torr, 65° C./0.3 Torr and 120° C./0.15 Torr and therefore are materials capable of becoming CVD film-forming materials for hafnium-series insulating films. The gate insulating film is positioned at a bottom portion of a semiconductor device, and the gate insulating film of the next-generation semiconductors becomes an ultra thin film having a film thickness of several nanometers. Thus, impurities in the gate insulating film have an extremely large influence on the electric characteristics of semiconductors. Therefore, a hafnium film-forming raw material therefor is required to be a high-purity product having an extremely low impurity concentration.
It is known to produce hafnium tetra-tertiary-butoxide Hf(OC4H9)4, tetrakis(acetylacetonato)hafnium Hf(C5H7O2)4, tetrakis(1-methoxy-2-methyl-2-propanolato)hafnium Hf(C5H11O2)4, hafnium tetra-tertiary-amyloxide Hf(OC5H11)4, tetrakis(3-methyl-3-pentoxy)hafnium Hf(OC6H13)4, or tetrakis(hexafluoroacetylacetonato)hafnium Hf(C5F6HO2)4 by obtaining a hafnium amide from hafnium tetrachloride and lithium alkylamide as starting materials and then reacting the hafnium amide with tertiary butanol C4H10O, acetylacetone C5H8O2, 1-methoxy-2-methyl-2-propanol C5H12O2, tertiary amyl alcohol C5H12O, 3-methyl-3-pentanol C6H14O, or hexafluoroacetylacetone C5F6H2O2 (see J. C. Bailar, H. J. Emeleus, Sir Ronald Nyholm, and A. F. Trotman-Dickenson, “Comprehensive Inorganic Chemistry”, Pergamon Press Ltd. pp. 462-475 (1973); R. C. Mehrotra, “Inorganic Chimica Acta Reviews”, Vol. 1, pp. 99-112 (1967); and Paul A. Williams, John L. Roberts, Anthony C. Jones, et al., “Chem. Vap. Deposition”, Vol. 8, pp. 163-170 (2002)).